For normal conducting cables of an OF cable, a POF cable, a CV cable, etc., a sample test using a spot sample and a frame test applied to the full length of a product are conducted as a method of checking the insulation performance of each cable.
The former is intended for checking to see if no problem arises if a voltage is continuously applied to the cable for a long time or checking to see if the cable has a predetermined insulation performance against a lightning surge on a spot sample.
In contrast, the latter is intended for checking that the cable has sufficient performance for system power-frequency overvoltage at the full length without damage to the cable as a shipment test.
In other words, it can be said that a check is made on the sample test to ensure that the cable manufacturing condition does not involve any problem and a check is made on the frame test to ensure that the manufacturing condition is carried out reliably at the full length of the cable.
On the other hand, research of a superconducting cable is made toward practical utilization. By way of example, a polyphase superconducting cable of multicore batch type having a plurality of cable cores in batch is proposed (for example, patent document 1: JP-A-2003-9330 at FIG. 5). FIG. 1 is a sectional view of a three-phase superconducting cable of three-core batch type. The superconducting cable 100 has three cable cores 110 twisted and stored in a heat insulation tube 120.
The heat insulation tube 120 is implemented as a double tube made up of an inner tube 121 and an outer tube 122 and has a heat insulation material (not shown) between both the tubes with the gap evacuated. Each cable core 110 includes a former 10, a conductor layer 20, an insulating layer 30, a shield layer 40, and a protective layer 50 from the center to the outside. The conductor layer 20 is formed by spirally winding multiple layers of a superconducting wire around the former 10, and the insulating layer 30 is formed by winding semisynthetic insulating paper. The shield layer 40 is formed by spirally winding a superconducting wire like that of the conductor layer 20 around the insulating layer 30. A refrigerant such as liquid nitrogen is filled into and circulated in the former 10 and the space formed between the inner tube 121 and the core 110 and the insulating layer is impregnated with refrigerant; this state is the use stage of the cable.
However, since the superconducting cable is not filled with a refrigerant at factory shipment, a method of appropriately testing the insulating characteristic with the superconducting cable filled with a refrigerant is not established.
For example, for a POF cable, insulating paper forming the insulating layer of the cable core is impregnated with high-viscosity insulating oil and the cable is protected so that the insulating oil is hard to flow out into the outside and then a frame test is conducted in a factory. At this time, the insulating layer contains air. However, after shipment, the cable core is pulled into a steel pipe and low-viscosity oil is filled into the steel pipe and a high pressure is applied, whereby the air existing in the insulating layer is dissolved into the insulating oil for placing the cable in a state in which the cable has the essential insulating performance. Therefore, the POF cable has the insulating layer impregnated with the insulating oil at factory shipment and a frame test of the cable can be conducted in a state close to the use state of the cable.
On the other hand, the superconducting cable is not filled with a refrigerant at factory shipment and is subjected to termination treatment in the laying field and then a refrigerant is filled into and circulated in the superconducting cable for placing the cable in the normal use state. Thus, a significant frame test cannot be conducted at factory shipment at which the insulating characteristic largely differs from that at the use time. If it is assumed that a capital investment to fill and circulate a liquid refrigerant from both terminals of the superconducting cable is conducted and a frame test is conducted with the superconducting cable placed in a cool state before shipment, the capital investment amortization and the costs for cooling heavily increase the cost of the superconducting cable. If the superconducting cable is cooled in a state in which it is wound around a drum, a mechanical stress acts in a more severe bend state than in laying in the field and there is also a possibility that damage to the cable may occur.
It is therefore a principal object of the invention to provide a dielectric strength test method of a superconducting cable that can evaluate the insulation characteristic of a superconducting cable in a state in which a refrigerant is filled without filling a refrigerant.